Time and thermostatically controlled switching arrangement



May 31, 1949. s. c. SHEPARD 2,471,457

TIME AND THERMOSTATICALLY CONTROLLED v SWITCHING ARRANGEMENT I Filed July 17, 1944 3 Sheets-Sheet 1 FIG.

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TIME AND THERMOSTATICALLY CONTROLLED SWITCHING ARRANGEMENT Filed July 17, 1944 s Sheets-Sheet 2 Inventor Sfanby (bm'm $496M! y Attorne May 31, 1949. s. c. SHEPARD 2,471,457

TIME AND THERMOSTATICALLY CONTROLLED SWITCHING ARRANGEMENT Filed July '17, 1944 :s Sheets-Sheet s INVEN TOR. STANLEY C'A/PflE/V SHfPA/Pz? Patented May 31, 1949 TIME AND THERMOSTATICALLY CON- TROLLED SWITCHING ARRANGEMENT Stanley Car-den Shepard, London, England, as-

signor, by mesne assignments, to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application July 17, 1944, Serial No. 545,329 In Great Britain July 30, 1943 8 Claims. 1

The present invention relates to time controlled electric circuits adapted for performing operations automatically according to a time schedule, or for like purposes.

For example, it may be desired to switch on a current and then perform an operation after a specified number of seconds, or to carry out alternately two functions at specified intervals of time. The present invention provides arrangements of this kind in which the timing is controlled by the heating and cooling of thermistors in appropriate circuits, and may be adjusted by simple means capable of calibration, so that the timing may be set as desired.

Thermistors are thermosensitive resistance elements having a temperature coeflicient of resistance which may be either positive or negative and which is moreover many times the corresponding coefficient for a pure metal such as copper. This property renders thermistors particularly suitable for a variety of special applications in electric circuits.

Various different materials are available for the resistance element of a thermistor, these various materials having difierent properties in other respects; as one example, a resistance material having a high negative temperature coefficient of resistance comprises a mixture of manganese oxide and nickel oxide, with or without the addition of certain other'metallic oxides, the mixture being suitably heat treated.

'I'hermistors have been employed in two difierent forms: (a) known as a directly heated thermistor and comprising a resistance element of the thermally sensitive resistance material provided with suitable lead-out conductors or terminals, and (1)) known as an indirectly heated thermistor comprising the element (a) provided in addition with a heating coil electrically insulated from the element. A directly heated thermistor is primarily intended to be controlled by the current which flows through it and which varies the'temperature and also the resistance accordingly. Such a thermistor will also be affected by the temperature of its surroundings and may therefore be used for thermostatic control and like purposes with or without direct heating by the current flowing through it. An indirectly heated thermistor is chiefly designed to be heated by a controlling current which flows but not necessarily, be different from the current which flows through the resistance element, but this type of thermistor may also be subjected to either or both of the types of control applicable .to a directly heated thermistor.

More detailed information on the properties of thermistors will be found in an article by G. L. Pearson in the Bell Laboratories Record, December, 1940, page 106.

According to the invention, there is provided an electric switching circuit comprising a source of electromotiv force, a thermistor arranged to control the current supplied from the source to a relay having an armature capable of assuming an operated or a released position, switching means adapted to be operated in order to supply heating current to the thermistor and adjustable means for controlling the heating current in such a manner that on operation of the switching means, the relay armature is caused to change from one position to the other after a specified delay determined by the setting of the adjustable controlling means.

According to another aspect, the invention provides an electric switching circuit comprising means to connect a source of electromotive force thereto, a relay deriving operating current from the source, an indirectly heated thermistor having its resistance element so disposed as to con trol the relay operating current, and adjustable means for controlling current supplied from the source to the thermistor heating coil in such a manner that on connecting the source to the switching circuit, the relay operates after a specified delay determined by the setting of the adjustable current supply means.

According to a further aspect, the invention consists in an electric switching circuit comprising a relay adapted to be periodically operated I and released, a thermistor arranged to control operating current for the relay derived from a source of electromotive force, a switch controlled directly or indirectly by the relay and adapted on being closed to connect heating current from the source to the thermistor, and adjustable means for controlling the heating current in such a manner that the switch remains closed for alternate periods whose duration is deterperature coeificient of resistance.

mined by the setting of the adjustable controlling means.

According to a final aspect, the invention covers an electrical switching circuit adapted to switch periodically from one condition to another, comprising a thermistor arranged to control the current supplied to a relay from a source of electromotive force and separate means for controlling the heating and cooling of the thermistor, respectively, the said separate means being each adjustable in such a manner that the times during which the circuit remains in the two conditions are independently determined by the settings of the respective adjustable means.

Figs. 1 to 5 of the drawings show schematic circuit diagrams of various embodiments of the invention according to which the timing of switching functions is controlled by the heating and cooling of thermistors; Various resistances other than thermistors are shown.

Fig. 1 shows a simple circuit according to the invention, which is adapted for performing an operation a predetermined time after switching on a current. In this figure, X and Y are terminals to which a source of alternating or direct current (not shown) is intended to be connected. By closing a key K the current may be supplied to some kind of device represented by Z. Two resistances R5 and Rs are connected in series between X and Y, Rs being shunted by an adjustable resistance R7. To the junction point of R5 and R6 is connected one terminal of the resistance element R1 of an indirectly heated thermistor T1, the other terminal being connected through the winding of a rely A to an adjustable tap on the resistance R5. The heating coil 11 of the thermistor is connected across an adjustable portion of the resistance Rs as shown.

The thermistor should have a negative tem- Assume first that the heating coil T1 is temporarily disconnected. Then when the key K is closed, a current will flow through the resistance element R1 and will heat the thermistor, reducing its resistance. Suppose, for the sake of example, that the circuit is required to operate from 1 to 10 seconds after closure of the key. Then R5 is adjusted so that in 10 seconds the resistance R1 has become reduced sufiiciently for the relay A to operate. Next, the heating coil 11 is reconnected, and the adjustable contact on the resistance R6 is moved to its extreme right hand position. The thermistor is now heated also by the current in the heating coil, and the resistance R1 will fall much more rapidly. The resistance R1 is now adjusted so that the relay A operates in exactly one second after the key K has been closed.

As the adjustable contact in R6 is moved towards the left hand end, so the relay will operate increasingly longer after closure of the key K. When the contact is at the extreme left hand position, no current flows through the reating coil 11 and the time interval is therefore 10 seconds. Hence any interval between 1 and 10 seconds may be obtained by appropriate adjustment of Re, which may be provided with a scale calibrated in seconds if desired. After the settings of R5 and R7 have been determined in the manner described, they do not require any subsequent adjustment.

The relay A may be provided with any desired combination of contacts such as a connected in any suitable way as indicated to the device Z (or to any other device) in order to initiate or control some operation a certain number of 4 seconds after the closing of key K, according to the setting of Re.

It will be evident that time should be allowed for the thermistor to cool before the key I is closed a second time, otherwise the operating time will be affected. Such cooling time for the example chosen might be about 10 seconds.

In an actual practical example of Fig. 1, the I resistance R1 of the thermistor when cold was of the order of 15,000 ohms and that of the heating unit 11 was ohms. A direct current source giving a voltage of 220 was connected to X and Y. The resistance R5 comprised, in order, a 220 volt 15 watt lamp connected to X, an adjustable resistance of 100 ohms and a fixed resistance of 200 ohms. R6 and R7 were adjustable resistances of 500 ohms each, and the resistance of the relay A was 50 ohms. This circuit save a range of operating time from 1 to 10 seconds.

In Fig. 2 there is shown an arrangement operating similarly to Fig. 1, but providing two alternative timing ranges. The thermistor T1 has its resistance element R1 connected in series with the relay A across an adjustable portion of a resistance R15 connected in series between the terminals X and Y to which the operating source is connected when the switch K is closed. A two-position switch having two mechanically coupled portions S1 and S2 is provided. When set in the position 1, as shown in Fig. 2, the heating coil 11 of the thermistor T1 is connected from the switch K through the resistance element R22 of a second thermistor T22 to the movable contact on resistance R15. The heating coil T22 of T22 is connected in series with adjustable resistance R16 to the terminals X and Y. The device Z is connected in thesame way as in Fig. 1. Both thermistors should have negative temperature coefiicients and the contact on resistance R15 is adjusted so that if the heating coil 122 be supposed to be temporarily disconnected, the relay A just fails to operate when the key K is closed.

It will be evident that temperature of the thermistor T1 now depends on the temperature of T22, which is directly heated by the current which flows through the heating coil 11. If the heating coil r22 now be supposed to be reconnected, the temperature of T22 is further raised at a rate depending on the adjustment of the resistance R16, increasing the temperature of T1 until the relay A finally operates. It will thus be seen that the introduction of the thermistor T22 delays the indirect heating of thermistor T1, and this enables much longer delays in the operation of the relay A to be obtained.

If the switch S1, S2 be placed in the position 2, the thermistor T22 is cut out and the heating coil n is connected directly to the resistance R16. The circuit is then substantially equivalent to Fig. 1 and differs therefrom only in the manner in which the current for the reating coil 11 is derived and controlled, the arrangement being more suitable when the source connected to X and Y is a low potential source.

The switch S1, S2 thus provides a means whereby two timing ranges may be obtained from one circuit arrangement. Clearly, however, if only one range is required, the switch could be omitted and appropriate permanent connections established corresponding to either position of the switch. In the case of position 2, of course, the thermistor T22 would be omitted.

In an actual practical example of Fig. 2, the source connected to terminals X and Y was a 6-volt battery, the resistance R being 200 ohms.

The two thermistors T1 and T2: were of the same type as T1 in the example previously quoted from Fig. 1, and the relay A was also the same. The following table gives the approximate time delays in the operation of relay A obtained for various values oi. Rm:

Time delay in secs. ior switch S 8 set in- Value of Ru, Ohms Position 1 Position 2 H 352 can QIFM , tive temperature coeflicients of resistance, are

connected in series to the junction point of Re and Re and to an adjustable contact on Rs through the winding of a relay B.

A second relay C is connected between X and Y through a pair of contacts b which are closed by the operation of relay B. The heating coil r: of

T: is connected across anadjustable portion of 39' through the closed contacts c1, and the heating coil 1': of T3 is connected across an adjustable portionoi R10 through the open contacts 02. The contacts 01 and c: are respectively opened and closed on the operation of relay C.

The circuit operates in the following manner:

As soon as the current source is switched on, the thermistor T2 heats up directly and indirectly, and T: directly only, until the combined resistance is reducedsufliciently to allow relay B to operate. B then operates C by closing the contacts b, and C disconnects the heating coil 1'2 and connects r3. T2 now cools and T3 heats up so that the two thermistors work in opposition, so that T: cools at a faster rate than T3 heats up so that there is a netincrease in Rz+R3. Presently relay B releases again on account of the reduction in the current, and the cycle is repeated.

It will be appreciated that the thermistor T3 delays the change caused by the cooling of T2 and the amount of delay depends on the adjustment of the contact on R10. Similarly, the rate of heating of T2 is determined by the adjustment of R9. R9 and R10 therefore control independently the times during which relay Bis released and operated respectively. These times will be called the off and on times respectively. If the contacts of R9 and R10 are both in the extreme right hand position, both the heating coils are ineffective and therefore the oif time will be a maximum and the on time a minimum. In the preliminary adjustment of the circuit, the contact on the resistance Re is set so that under this condition the current through the relay is not suincient to operate it. On adjusting the contact on Re towards the left hand end, a small heating current will flow so that the resistance R: is reduced a little further causing the operation of relay B after a time which is the shorter as the contact on Re is nearer the left hand end. It will be evident that the maximum 011 time'will depend on the adjustment of Ra, being increased as the contact is moved towards the left hand end.

The minimum on timcs depends on the setting chosen for the contact of Re, and by adjusting the contact of R1o-towards the left, the on time may be increased without appreciably affecting the oil time.

The resistances R9 and R10 may be provided with scales calibrated to read the corresponding off and on times, and Rs having been adjusted for the initial condition does not in general require any further change.

Either or both of the relays B and C may be provided with extra combinations of contact springs (not shown) for the purpose of switching or controlling the operations of some device.

The two thermistors T2 and T3 need not be alike, and in fact R2 will preferably be greater thah R3. I

Fig. 4 shows a modification of Fig. 3, which operates in a slightly different manner, and moreover includes a delaying thermistor with a switch in order to obtain two ranges in the same way as described in connection with Fig. 2. It comprises three resistances R11, R12 and R13 connected between the terminals X and Y and corresponding respectively to Rs, R10 and R9 of Fig. 3. Relays D and E correspond respectively to B and C, no thermistor corresponding to T3. The thermistor T4 corresponds in function to T2, and should have a negative temperature coeiiicient of resistance.

Relay D may be provided with a compensating winding g in addition to the normal operating winding 1, as shown. The winding g is connected 'in an optional auxiliary circuit which will be described later, but which will for the present be disregarded.

The double range switch S1, S2 is arranged in substantially the same way as in Fig. 2. Assuming it to be set in the position 2, as shown, in order to obtain the short timing ranges, it will be seen that the resistance element R4 of T4 is connected to the junction of resistances R11 and R12 through the normally closed contacts e1 controlled by relay E. The heating coil 1'4 is connected across a variable portion of R13 through the portions S1 and S2 of the switch and through the normally closed contacts e2 also controlled by relay E. Relay E is connected between the terminals X and Y through the normally open contacts d controlled by relay D. 1

It will be seen that the thermistor is heated directly by current through the operating winding 1 of relay D, derived from the potential across part of R11, and it is heated indirectly by current derived from the potential across part of R13. When relay D operates, relay E is operated by the closing of contacts d and opens both e1 and er. The latter cuts oil the current of the heating coil 1'4, but the former increases the direct heating current by introducing part of the resistance which depends on the rate of heating of T4, can be controlled by adjustment of the contact on R12; and that the on time depends on the rate of cooling of T4 and can be controlled independently by adjusting R12. The circuit of Fig. 4 then behaves substantially in the same way as that of Fig. 3 and is initially set similarly by adjusting Rn so that the relay D does not operate when the contact in R1; is set at the extreme right hand side. The maximum 01? time is increased, as before, by moving the contact of R11 towards the left hand side. The resistances R12 and R13 (which correspond respectively to Rio and R9 of Fig. 2) may be provided with calibrated scales.

By operating the switch S1, S2 to the position 1, an additional indirectly heated thermistor Ta:

(which should have a negative temperature efiicient of resistance) is introduced to delay the heating and cooling of T4, and is arranged in a similar manner to that described with reference to T22 in Fig. 2, for the purpose of lengthening the on and OK times.

It will be seen that the heating current through n is now controlled by the resistance element R3; of T33, the heating current through r23 being derived from the resistance R13. Thus, it will be seen that before relay D operates, the heating of T4 is delayed by the heating of T23, and when relay D operates causing the operation of E, the contacts 61 and 62 open as before. The cooling of T4 is delayed both by the introduction of the portion of resistance R12 as previously explained, and by the cooling of T33, the indirect heating current of which is cut off by the opening of 02. In this way the on periods may range from, say, 3 to 30 seconds, and the off periods from perhaps 30 to 90 seconds.

It may be added that in Figs. 3 and 4 the relays C and B respectively could be omitted if desired, their contacts being transferred to the relays B and D respectively. It isf however, preferable to retain them, because the accurate timing of the circuit depends on the constancy of the operate and release currents of relays B and D, which is likely to be better maintained if these relays do not have to operate more than a single pair of contacts. Moreover, the relays C and E for other reasons might have to be of a type not adapted for marginal operation; for example, they might be mercury relays intended for switching relatively large currents. For a similar reason it might not be convenient or practicable to employ the relay A in Fig. l or 2 to carry out the desired switching operation directly, and it would then be better to provide it with a single pair of contacts for operating a switching relay (not shown) or appropriate type carrying as many sets of contacts as desired.

Any of the thermistors shown in Figs. 1, 2, 3 or 4 could be replaced by two or more indirectly heated thermistors with their resistance elements and heating coils respectively connected in series (or in parallel). Such groups of thermistors would behave substantially like a single thermistor, but might give greater flexibility of design of the circuit ifthe number of different types of thermistors available should be limited. Likewise, the various adjustable resistances shown could partly comprise fixed resistances arranged so that convenient ranges of adjustment are obtained with suitably open scales. Such modifications are, of course, well understood.

Thus in a certain particular example of Fig. 3, the thermistor T2 was actually represented by three similar thermistors having their resistance elements and their heating coils, respectively, connected in series, and T3 was a single thermistor of the same type. The circuit was designed so that the on period could be varied from 1 to 8 sec- .onds and the off period from 3 to 15 seconds.

In a practical case of Fig, 4, in which the thermistor T33 was not used, the connections being as for the switch S1, S2 in the position 2, and intended for approximately the same ranges of on and off times as those just quoted, T4 was represented by two similar serially connected thermistors, the cold resistance of each of which was of the order of 15,000 ohms, the resistance of the heating coils being about ohms each. A direct current source wasconnected to terminals X and Y, the voltage being about 220. Resistance R11 comprised in order a 220 volt 15 watt lamp connected to terminal Y, an adjustable resistance of 100 ohms and a fixe'd resistance of 200 ohms. Resistances R12 and R13 were adjustable resistances of 100 ohms and 500 ohms respectively. The winding j of relay D had a resistance of 1500 ohms. E was a relay having a resistance of about 15,000 ohms.

Referred again to Fig. 2, a third indirectly heated thermistor (not shown) could be associated with T22 in substantially the same way as T22 is associated with T1, and this would enable still longer operating delays to be obtained; and clearly the same process could be continued indefinitely by connecting a chain of any number of thermistors in the same manner. It is evident that the same principle could be adapted also with the arrangement of Fig. 4.

Likewise, the currents through the heating coils of either, or both, of the thermistors'T2 and T2 in Fig. 3 could be controlled indirectly through an individual thermistor or chain of thermistors arranged substantially as described.

It will be evident that unless the thermistors employed in any of the switching circuits described are keptin a constant temperature enclosure, the timing is liable to be efiected by changes in the air temperature. Thus, for example, a rise in temperature will reduce the resistance of the thermistors and will lengthen the on periods and shorten the oif periods. The effect may be negligible if the timing periods are not too long. It can, however, be very easily compensated automatically as shown in Fig. 4 by providing the relay D with an extra compensating winding g which is connected to the operating source in series with a suitable directly heated thermistor T5 having a negative temperature coeflicient, placed in close proximity to the other thermistor or thermistors. The current in the compensating winding g should be arranged to oppose the efiect of the current in the operating winding f and should be adjusted so that when the air temperature rises, the opposing current is increased (on account of the consequent reduction in resistance of the directly heated thermistor) by an amount which compensates for the corresponding increase in the operating current arising from the slightly higher mean temperature of the indirectly heated thermistors. If the operating source is alternating, the same arrangement can be adopted, provided that the compensating current is supplied in opposite phase to the operating current, and it may be necessary to provide phase correcting means in any appropriate way to secure this. A variable resistance R14 may be included as shown to facilitate the adjustments. It will be obvious that relays A and B in Figs. 1, 2 and 3 could be equipped with compensating windings and associated circuits in exactly the same way.

By a variation of this compensating arrangement, the circuit of Fi 3 or 4 could be adapted to control the heating of an oven or water bath,

or the like. As shown in Fig. 5, the circuit is arranged to switch on the heater for the oven during the oil periods and to switch it off during the on periods. The controlling directly heated thermistor T5 is placed in the oven, as represented by dashed lines K, but the connections of the controlling winding 9 of the relay D (or B) are reversed, so that the controlling current aids instead of opposes the operating current. Thus, as the temperatur of the oven rises the action of the controlling thermistor will cause the periods during which the heater is switched on to be continually decreased and the periods during I term thermistor in which it is switched off to be increased until a temperature is reached at which it is switched on just long enough in each period to balance the heat losses, and the arrangement stabilises at that temperature. The adjustable resistance R14 could be used to vary the stabilising temperature.

Although in the embodiments described the thermistors have been specified as having negative temperature coefiicients of resistance, the circuits can be easily adapted to use the thermistors with positive temperature coefllcients. Referring to Fig. 1 or 2, the resistance element R1 01' the thermistor T1 could in that case be connected in parallel with the relay A instead of in series, an appropriate resistance being connected it necessary in the place of R1. The circuit having been adjusted so that the relay A operates in 10 seconds (for example) when the heating coil n is disconnected, the increase in the value of R1 as the thermistor heats up will permit the relay to operate earlier, according to the setting of the contact in Re. Alternatively, Fig. 1 or 2 may be used with a thermistor or thermistors having a positive temperature coeflicient without any alteration except that the relay A is arranged to operate immediately on closing the key K, and to be released by the increase in R1 as the thermistor heats up, for example in from 1 to 10 seconds according to the setting oi. Re. Likewise, in Figs. 3 and 4, if the thermistors have positive temperature coeflicients, the two series connected resistance elements (or the single element R4 in the case of Fig. 4) are connected to shunt the corresponding relay B (or D) being replaced if necessary by a suitable resistance. The circuit-s then operate in the same way as before, except that the relay is operated by the increase in the shunt resistance instead 01 by the decrease in the series resistance, when the thermistors heat up. Similarly, in Fig. 4, T5 could have a positive temperature coeiiicient if it were arranged to shunt the winding g.

Alternatively, by changing the contacts I) or at so that they are normally closed, but are opened when the corresponding relay operates, the circuits of Figs. 3 and 4 employing thermistors with positive temperature coeiiicients will operate practically as described without any other change. The only difference is that the relays B and D will remain operated while the thermistors are heating up instead of while they are cooling. The circuits may obviously be adapted for thermistors with positive temperature coefilcients by making minor change in various other ways. The circuits which have been described are capable of various modifications of the connections. For example, the heating coil n of the may be represented by a series or parallel con-- nected group or thermistors which behav collectively in the same way as a single thermistor, the the claims which follow is to be understood to include such a group of thermistors.

What is claimed is:

1. An electric switch periodically from one condition to another, including a source of electromotive force,"

a relay capable of being periodically operated and released for establishing connections for one condition when operated and for the other condition when released, a thermistor connected to said source to supply current to said relay to actuate the relay, adjustable means for controlling the heating of the thermistor, adjustable means for controlling the cooling of the thermistor, means for alternately connecting said two last-named means to affect the thermistor upon actuation of the relay, and a second thermistor connected to control the duration of the periods during which the relay remains operated and remains released in accordance with the temperature at the location oi the second thermistor.

2. An electric thermostatic control switching ing said two last-named means to affect the circuit for controlling temperature within a confined space by compressing a heating source, a source of electromotive force, a relay capable of being periodically operated and released to switch the heating source on and off, a thermistor connected to said electromotive source to supply current to said relay to actuate the relay, adjustable means for controlling the heating of th thermistor, adjustable means for controlling thecooling of the thermistor, means for alternately connectthermistor upon actuation of the relay, means for compensating for variations in the performance of the circuit consequent on changes in the temperature of the space surrounding the thermistor, and a second thermistor connected to control the duration of the periods during which the relay remains operated and released in accordance with the temperature of said space.

3. An electric switching circuit including a source of electromotive force, a relay capable of being periodically operated and released, a thermistor connected to control the operating current of said relay as derived from said electromotive force, a switch controlled by said relay and connected so as, on being closed, to connect heating current from said source to said thermistor, adjustable means for controlling said heating current whereby said switch remains closed for alternate periods, the duration oi which is determined by the thermistor characteristics and setting of said adjustable heating current controlling means, and adjustable means further comprising an adjustable resistance individual to said thermistor and connected in series with said source of electromotive force, and means for applying the electromotive force across an adjustable portion of said resistance to provide heating current for said thermistor.

4. An electric switching circuit including a source of electromotive force, a relay deriving switching circuit adapted to 11 operating current from said source, an indirectly heated thermistor having the resistance element thereof so connected as to control the relay operating current, adjustable means for controlling current supplied from said source to the thermistor, heating coils whereby on connecting said source to said'switching circuit, said relay operates after a specified delay determined by the thermistor characteristics and the setting of said adjustable current supply controlling means, said adjustable means further comprising an adjustable resistance individual to said thermistor and connected in series with said source of electromotive force, means for applying, potential across an adjustable portion of said resistance so as to provide heating current for the thermistor. and an adjustable resistance and means for connecting the last mentioned resistance in shunt with said individual resistance.

5. An electric switching circuit including a source of electromotive force, a relay deriving operating current from said source, an indirectly heated thermistor having the resistance element thereof so connected as to control the relay operating current, adjustable means for con trolling current supplied from said source to the thermistor, heating coils whereby on connecting said source to said switching circuit, said relay operates after a specified delay determined by the thermistor characteristics and the setting of said adjustable current supply controlling means, said adjustable means further comprising a resistance individual to said relay, said resistance being connected in series with said source of electromotive force whereby the current for operating said relay is derived from the potential across an adjustable portion of said individual resistance.

6. An electric switching circuit including a source of electromotive force, a relay capable of being periodically operated and released, a thermistor connected to control the operating current of said relay as derived from said source of electromotive force, a switch controlled by said relay connected so as, on being closed, to connect heating current from said source to said thermistor, adjustable means for controlling said heating current whereby said switch remains closed for alternate periods, the duration of which is determined by the thermistor characteristics and the setting of said heating current controlling means, means for compensating for the variations in performance of the circuit consequent on changes of the mean temperature of the space in which the thermistor or thermistors is or are placed, said compensating means ,comprising a compensating winding on said relay, means for passing a current through said compensating winding from said source of electromotive force in such a direction as to oppose the effect of the current in the operating winding of the relay, and the compensating thermistor placed in said space and arranged to control the compensating current in accordance with the mean temperature of said space.

'7. An electric thermostatic control switching circuit operating to switch periodically from one condition to another for controlling the tem perature within a confined space comprising a source of electromotive force, a relay capable of being periodically operated and released for establishing connections for one condition when 12 operated and for the other condition when released, a thermistor arranged to control a current supplied to said relay from said source of electromotive force to actuate the relay in accordance with changes in the temperature of the thermistor, adjustable means for controlling the heating of said thermistor, adjustable means for controlling the cooling of said thermistor, respectively, whereby the times are independently determined by the characteristics of the thermistor and the setting of said adjustable means, a heating source,- and means for automatically varying the on and off periods of said heating source in accordance with the mean temperature within said confined space.

8. An electric thermostatic control switching circuit operating to switch periodically from one condition to another for controlling the temperature within a confined space comprising a source of electromotive force, a relay capable of being periodically operaoted and released for establishing connections for one condition when operated and for-the other condition when released, a thermistor arranged to control the current supplied to said relay from said source of electromotive force to actuate the relay in accordance with changes in the temperature of the thermistor, adjustable means for controlling the heating of said thermistor, adjustable means for controlling the cooling of said thermistor, respectively, whereby the times during which the thermistor remains in said two positions are independently determined by the characteristics of the thermistor and the setting of said adjustable means, means for automatically varying the on and off periods of said heating source in accordance with the means temperature within said space, said last mentioned means comprising a controlling winding on said relay, means for passing current through the controlling winding from .said source of electromotive force in such a di- -rection as to aid the effect of the current in the operating winding of the relay, and a controlling thermistor placed within said space and connected so as to adjust the current in said controlling winding in accordance with the means temperature of said space.

STANLEY GARDEN SHEPARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,034,881 Scheer Mar. 24, 1936 2,112,542 Myers Mar. 29, 1938 2,148,491 Moore Feb. 28, 1939 2,244,058 Crafi'ord June 3, 1941 2,304,613 Vroom Dec. 8, 1942 2,318,358 Bedford May 4, 1943 FOREIGN PATENTS Number Country Date 397,158 Germany June 14, 1924 557,707 Great Britain Dec. 1, 1943 640,150 Germany Dec. 23, 1936 OTHER REFERENCES Bell Laboratories Record, December, 1940, pages 106-111. 

